This invention relates generally to equipment supports, and more particularly to portable equipment utilized in conjunction with motion picture or video cameras.
Mobile film or video cameras typically require angular and spatial stability in order to obtain smooth, high-quality results. The Steadicam® portable camera stabilizing device, which has become a de facto standard in the TV and movie industry, was developed to permit stable ambulatory videography or cinematography by an operator. The inventor's U.S. Pat. No. 4,017,168 (Re. 32,213), U.S. Pat. Nos. 4,156,512 & 4,474,439 are directed to aspects of such stabilizing devices.
Spring powered ‘equipoising’ parallelogram arms have been used for decades to support and position payloads such as lamps, x-ray machines and dental equipment. These arms rely to a greater or lesser extent on friction to retain a set angle or position, since existing spring geometries do not necessarily provide appropriate or consistent lift throughout the entire angular excursion of the parallelogram links. The Steadicam®, however, provides near frictionless support of the floating camera payload in order to isolate a camera from unwanted spatial movements of the operator, and further mandates a spring design for the support arm that will equipoise almost perfectly, that counters the fixed weight of the gimbaled camera assembly with nearly constant positive buoyancy from its lowest to its highest point of parallelogram excursion
The formulas for determining the appropriate spring rate to achieve this in equipoising arms factor down to the expression K=P/d, where K is the spring rate, P is the load and d is the height of the lifting triangle, which is incorporated into the parallelogram and exercises it upward. When a spring of the rate specified in the above formula is deployed as a side of the triangle, it produces the appropriate force to exactly lift the specified weight throughout the entire vertical range of motion. This property is termed “iso-elasticity”.
It is noted that any shaped lifting structure can be used that follows the principals described herein and can be substituted for the “lifting triangle” referenced extensively throughout. It is also noted that reference to a triangle or structure sides does not necessarily mean the sides are physical structures.
In order to lift the load consistently throughout the entire excursion of the typical parallelogram arm, however, spring rate indicated by the above formula mandates spring designs that are typically up to three times as long as the diagonals across which they are to act. The present inventor's U.S. Pat. Nos. 4,208,028 and 4,394,075 originally solved this problem by dividing the spring into a chain of three spring segments in series, interconnected by steel cables running over pulleys at the parallelogram link ends, that permitted the entire spring to expand and contract, and yet still applied the sum of the collective force in series across the diagonal, as if produced by a single, continuous spring.
In practice it was found that when the support of lighter cameras required relaxing the tension of the spring series, the spring rate became inappropriate for those reduced loads and iso-elasticity was compromised. As a consequence, the arm tended to ‘ride’ harshly and the desired positive buoyancy for the load only prevailed in one sector of its vertical excursion. Further, this three-spring solution was complex and expensive, requiring a plurality of pulleys and robust cables.
The present inventor's U.S. Pat. No. 5,360,196 (the '196 patent) describes an arm that is powered by a single, high-rate spring, applying its force via a differential pulley and tackle through a cable running across the diagonal, so that the effective rate is appropriate for iso-elasticity according to the above formula. This arm adjusts the lifting strength of the arm in a novel manner by raising and lowering the attachment point of the spring cable within the parallelogram linkage (thus increasing or decreasing the height, and thus the efficiency, of the lifting triangle) without compromising the spring rate required to provide ‘iso-elasticity’. The same formula, K=P/d, indicates that if only the height of the appropriate lifting triangle is increased or reduced proportionately with the weight to be carried, the property of iso-elasticity will be maintained. In practice, the arm embodying the technology claimed in the '196 patent was found to be somewhat frictional due to the ‘gear ratio’ of the differential pulley. Also, the closer iso-elasticity was achieved, the more erratic was the arm's behavior at the extremes of high and low lifting position. As the lifted or depressed angle of an equipoising support arm exceeds 50° from the horizontal, its exact performance is increasingly subject to minute variations of load, torque, friction and the collective bearing tolerances of its pivots.
The present inventor's continuation of the above patent, U.S. Pat. No. 5,435,515 (the '515 patent), reverted to the complex and expensive ‘three-spring’ method to achieve iso-elasticity, but sought to achieve predictable performance at the high and low extremes of excursion by selectably decreasing the lifting efficiency of the spring geometry. This was done by adjustably offsetting the path of spring termination so that it was raised and lowered along a line within the parallelogram that was angularly displaced from vertical in order to slightly reduce the degree of iso-elasticity. The angle of the line was fixed, however, and since only its lateral displacement could be adjusted, its effect inappropriately increased rather than decreased the lifting efficiency where it was most needed—as the spring termination point was lowered.
What was needed was a means that would permit the use of a single spring that could actually fit within the diagonal distance of a support arm parallelogram and still produce iso-elastic equipoising of the load. An arm was also needed that would predictably, frictionlessly, equipoise the load throughout its entire excursion—all the way from its lowest to its highest parallelogram positions.
All previous Steadicam®-type arms, particularly those that approach iso-elasticity under certain loads, have needed to arbitrarily restrain their vertical travel to a degree of parallelogram excursion well short of maximum or minimum in order to avoid unruly, unpredictable performance at extreme high and low angles. Even with a degree of control over iso-elasticity, parallelogram arms were still prone to unexpected and forcible closure as angles neared 60° above or below horizontal. Arms would typically be characterized as those that ‘behaved’ and those that arbitrarily ‘locked up’ at those high and/or low excursions.
Restraining ‘bumpers’ have, therefore, been a feature of these equipoising arms from the beginning. The more ‘iso-elastic’ the spring geometry, the more irregularly the arms tend to lift at these vertical extremes of excursion. This is partly a consequence of the unpredictably varying torques imposed by the cantilevered, gimbaled payloads that hang at various angular positions relative to the arm parallelogram. The result has been an uncontrollable tendency to lock up, or lurch ‘over-centers’, at the high or low position, and so various bumper designs have, in some cases, restrained the travel to as little as 45° above level. In no case were the angular extremes of lift available from such parallelograms, and thus the lifting range of travel of the arms was curtailed. In addition, bumpers “bumped” more or less suddenly and further caused operators to be wary of approaching them—which further limited the usefulness of these support arms. What was needed was practical control of the general level of iso-elasticity, and further, some additional automatic control over the geometrical contour of lift that would provide smooth, predictable behavior at these extreme high-low angles of arm excursion, by gently de-powering the arms just before bumping, clunking or shooting over centers and locking up.
Applicant has previously refined the ‘offset’ concept described in the '515 patent, and placed it fixedly ‘outside’ vertical to, in effect, uniformly change the effective rate throughout the arm's excursion and simulate the effect of the correct rate using a spring short enough to fit into the diagonal (this concept has been successfully marketed as the ‘Flyer’ arm). Limitations in the Flyer arm, however, were evident at extremes of high lift. There was also an irregular curve of performance.
Parallelograms are capable of closing to nearly ±80°, but have previously been unusable at those angles due to the foregoing problems, despite various bumper schemes employed to tame these extreme up/down positions.
What is further needed is a way to regularize and level out the lifting curve and avoid the tendency to jump ‘over centers’ and lock up at high/low extremes.